1. Field of the Invention
The present invention generally relates to an organic light emitting display and a driving method thereof.
2. Description of Related Art
Recently, various flat panel display devices having reduced weight and volume in comparison to a cathode ray tube (CRT) have been developed. Examples of flat panel display devices include liquid crystal displays, field emission displays, plasma display panels, organic light emitting displays, etc.
Among these examples, the organic light emitting display displays an image utilizing organic light emitting diodes (OLEDs) that generate light by the recombination of electrons and holes. An organic light emitting display generally has a rapid response speed and a low power consumption.
FIG. 1 is a circuit diagram illustrating a pixel of a conventional organic light emitting display.
Referring to FIG. 1, a pixel 4 of a conventional organic light emitting display includes an organic light emitting diode OLED and a pixel circuit 2 that is coupled to a data line Dm and a scan line Sn to control the organic light emitting diode OLED.
An anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit 2, and a cathode electrode thereof is coupled to a second power ELVSS. The organic light emitting diode OLED generates light having a brightness (which may be predetermined) corresponding to a current supplied from the pixel circuit 2.
The pixel circuit 2 controls an amount of current supplied to the organic light emitting diode OLED in accordance with a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. To this end, the pixel circuit 2 includes a second transistor M2 coupled between a first power ELVDD and the organic light emitting diode OLED, and a first transistor M1 coupled to the second transistor M2, the data line Dm and the scan line Sn, and a storage capacitor Cst coupled between a gate electrode and a first electrode of the second transistor M2.
A gate electrode of the first transistor M1 is coupled to the scan line Sn, and a first electrode thereof is coupled to the data line Dm. And, a second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst. Herein, the first electrode is set as one of a source electrode or a drain electrode, and the second electrode is set as an electrode different from the first electrode. For example, if the first electrode is a source electrode, the second electrode is a drain electrode, and vice versa. The first transistor M1 coupled to the scan line Sn and the data line Dm supplies a data signal on the data line Dm to the storage capacitor Cst by being turned on when the scan signal is supplied from the scan line Sn. At this time, the storage capacitor Cst is charged with a voltage corresponding to the data signal.
A gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst, and a first electrode thereof is coupled to the other terminal of the storage capacitor Cst and the first power ELVDD. And, a second electrode of the second transistor M2 is coupled to an anode electrode of the organic light emitting diode OLED. The second transistor M2 controls an amount of current flowing from the first power ELVDD, through the organic light emitting diode OLED, to the second power ELVSS in accordance with a voltage stored in the storage capacitor Cst. At this time, the organic light emitting diode OLED generates light corresponding to the amount of current supplied by the second transistor M2.
In the conventional pixel 4, the second transistor M2 is driven as a substantially constant current source supplying a current (e.g., a predetermined current) to the organic light emitting diode OLED in accordance with the voltage stored in the storage capacitor Cst. Herein, the transistor M2 should be driven in its saturation region in order that the second transistor M2 drives a substantially constant current. Therefore, the voltage of the first power ELVDD and the second power ELVSS are set so that the second transistor M2 is driven in the saturation region.
In more detail, the voltage between the first power ELVDD and the second power ELVSS can be expressed as shown in the following Equation 1:ELVDD−ELVSS>Vds—sat+Voled+Vmt+Vmo   Equation 1
In Equation 1, Vds_sat represents a minimum voltage between the first electrode and the second electrode (e.g., the source and the drain) of the second transistor M2 for driving the second transistor M2 in the saturation region when a maximum current (i.e., the saturation current of the second transistor M2 when the data value representing the highest gray level is supplied on the data line Dm and stored in the storage capacitor Cst) flows from the pixel circuit 2 to the organic light emitting diode OLED. Voled represents a voltage applied to the organic light emitting diode OLED when the maximum current is supplied.
Vmt represents voltage margin due to a process deviation of the second transistor M2, and Vmo represents a voltage margin corresponding to the process deviation and the temperature characteristics of the organic light emitting diode OLED.
Actually, in the organic light emitting diode OLED, the voltage margin Vmo corresponding to the temperature changes even in the case where the same current is supplied. Therefore, Vmo is set such that the pixel 4 can be stably driven in consideration of the temperature characteristics of the organic light emitting diode OLED.
Meanwhile, when the voltages of the first power ELVDD and the second power ELVSS are set as shown in Equation 1, power consumption may be undesirably high. In particular, the voltage margin Vmo that is added in consideration of the temperature characteristics may result in 20% to 30% of the power consumption. Therefore, a method capable of reducing power consumption by lowering the margin voltage of Vmo is desired.